The present invention relates to a spark plug for an internal combustion engine which including an Ir (iridium) alloy firing tip connected to an opposed portion of at least one of a center electrode and a ground electrode opposed via a discharge gap intervening therebetween. Furthermore, the present invention relates to a method for manufacturing this spark plug. The present invention is applicable to various kinds of spark plugs used in automotive vehicles, cogeneration facilities, gas pressure pumps, etc.
In general, a spark plug has a center electrode, an insulator holding the center electrode, a housing holding and fixing the insulator, and a ground electrode having one end portion connected to the housing and the other end portion opposed to the center electrode via a discharge gap.
According to this king of spark plug, to assure long-lasting lifetime for satisfying the requirements of high performance and easy maintenance of the engines, an Ir alloy firing tip is disposed on a spark discharge portion of the center electrode or the ground electrode which is an opposed portion of the center electrode or the ground electrode facing to a discharge gap.
The Ir alloy is greatly different from the electrode base material (e.g., nickel alloy etc.) in thermal expansion coefficient. Hence, it is necessary to prevent the Ir alloy firing tip from falling off the electrode due to generation of a thermal stress. To this end, a laser welding is applied to form a fused layer between the Ir alloy firing tip and the electrode base material so that the fused layer has an intermediate thermal expansion coefficient compared with those of the Ir alloy and the electrode base material, thereby reducing the thermal stress acting between the Ir alloy firing tip and the electrode and assuring excellent bondability between them.
According to the laser welding method, the Ir alloy firing tip is integrated beforehand with the electrode base material by resistance welding or the like, and then a laser is irradiated onto an entire circumferential surface of the Ir alloy firing tip while the integrated assembly is rotated about the axis of the Ir alloy firing tip
In this case, the laser weldability is greatly influenced by the configuration of the firing tip and the electrode base material at each laser irradiation position. If the configuration of the firing tip and the electrode base material relative to the laser beam is not uniform at each laser irradiation position, the melting process of the welded portion will be different at each laser irradiation position. The bondability between the Ir alloy firing tip and the electrode cannot be assured. Accordingly, it is usual that the conventional Ir alloy firing tip is configured into a columnar or cylindrical shape so that the configuration of the firing tip is constant at each laser irradiation position when the firing tip is rotated about its axis during the welding operation.
However, configuring the Ir alloy firing tip into a columnar or cylindrical shape requires essentially a rolling process, a wiredrawing process, and many other processes (for example, refer to Japanese Patent No. 3000955 corresponding to U.S. Pat. No. 5,977,695).
Furthermore, to reduce the manufacturing costs, it is conventionally proposed to use an Ir alloy firing tip having a quadrangular or hexagonal configuration in the cross section taken along a plane perpendicular to the axis of the firing tip and being bonded to the electrode by laser welding (for example, refer to Japanese Patent No. 3000955 corresponding to U.S. Pat. No. 5,977,695).
However, according to the evaluations conducted by the inventors of this invention, the firing tip having a quadrangular or hexagonal configuration in the cross section has so small apical or face angle that a large stress is concentrated to the welding portion of the firing tip and the electrode due to the edge effect. Thus, the quadrangular or hexagonal firing tip cannot assure satisfactory bondability, compared with a columnar firing tip.